A head mount display (HMD), which is attached to a part of a user's body such as a head when being used, and which displays an image in front of the eyes of the user, is used in various fields including a field of virtual reality. Usually, the HMD shout out external light to exclusively display an image, and it is generally formed in a goggle-like configuration or in a large-eyeglass-frame-like configuration. When using the HMD, the user can see the image in whichever direction the user may face. On the other hand, the user cannot see the external world.
In recent years, there has been proposed a small image display unit which displays an image in a part of a field of vision, making it possible to see both the external world and the image.
In this type of image display unit, it is possible to selectively watch either the external world or the image through movement of the line of vision, so the display unit is valuable in that it can be used in a daily life situation and it does not matter whether it is used indoors or outdoors.
While this type of image display unit is intended for attachment to an existing eyeglasses, eyeglasses vary in terms of presence/absence of a frame and of the material, thickness, etc. of the frame. In view of this, there is a demand for development of a technique enabling to easily attach an image display unit to any type of eyeglasses.
In order for an image display unit to be easily attached to any type of eyeglasses, it is necessary for the image display unit to be detachable with respect to the eyeglasses.
Further, it is necessary for the mounting angle of the image display unit with respect to the eyeglasses to be adjustable.
In the course of technical development taking those necessities into consideration, the inventor of the present invention focused attention to the fact that the flat surfaces of eyeglass lenses are element common to all eyeglasses, thereby inventing an image display unit as disclosed in Japanese Patent Application No. 2004-233912.
An image display unit according to the above-mentioned invention has an intermediate member that can be fixed to the flat surface of an eyeglass lens, and an image display unit main body equipped with a display means situated in front of one eye of the user during use of the image display unit, and a fixation means allowing detachable fixation to the intermediate member.
FIG. 8 is a perspective view showing of an image display unit main body 100 and an intermediate member 800 of an image display unit 10 of this type.
As shown in the figure, the intermediate member 800 is equipped with a plate-like base 210, and a foundation 820 protruding forwards from a front surface of the base 210 (In this specification, the word “front” means the side farther from the face of the user wearing the eyeglasses on his face). The foundation 820 has a concave surface 821 open on the front side when it is fixed to the eyeglass lens. The concave surface 821 constitutes a joint portion of the intermediate member 800 with respect to the image display unit main body 100. Further, on the rear surface of the base 210, there is provided an adhesive layer 240 formed by an adhesive in the form of a layer. By the presence of the adhesive layer 240, it is possible to fix the intermediate member 800 to the eyeglass lens.
The image display unit main body 100 is equipped with a case 110 formed substantially as a rectangular parallelepiped. On the rear surface of the case 110, there is provided a lens barrel 120. The lens barrel 120 is formed substantially as a cylinder. Inside the lens barrel 120, there is provided a magnifying lens 130. On the rear surface of the case 110, there is further provided a convex surface 101 as a fixing means. The convex surface 101 protrudes rearwards from the case 110 and constitutes a part of a predetermined spherical surface. The convex surface 101 constitutes a joint portion of the image display unit 100 with respect to the intermediate member 800.
For example, both the convex surface 101 and the concave surface 821 may be formed of magnets, or one of them may be formed of a magnet and the other of metal. That is, the convex surface 101 and the concave surface 821 are attracted and fixed to each other by a magnetic force.
FIG. 9 shows how the convex surface 101 of the image display unit main body 100 and the concave surface 821 of the intermediate member 800 are attracted and fixed to each other.
With the construction of the joint portion of the image display unit 10, it is possible to easily attach the image display unit 10 detachably to an eyeglass lens GL of any type of eyeglasses. Further, to change the mounting angle of the image display unit main body 100 with respect to the intermediate member 800 during use of the image display unit 10, it is possible to cause the convex surface 101 to slide with respect to the concave surface 821, with the intermediate member 800 and the image display unit main body 100 fixed to each other.
That is, the construction of this joint portion is used as a universal joint.
However, this universal joint is not without its problems to be solved. That is, the thickness of the end portion of the concave surface of this universal joint is larger than the thickness of the central portion thereof, and more magnetic lines of force are distributed on the outer side than in the inner side of the convex surface and the concave surface. The magnetic lines of force pass from the concave surface to the convex surface (or from the convex surface to the concave surface), and pass through the air from the convex surface (or from the concave surface) before returning to the concave surface (or the convex surface). The more magnetic lines of force existing on the outer side, the weaker the attracting force.
Further, due to their structures, the convex surface and the concave surface are attracted and fixed to each other firmly at the peripheral edge portion of the concave surface. The direction of the vector of the attraction force is an oblique one directed toward the central portions of the convex surface and the concave surface, so if the convex surface is slightly shifted with respect to the concave surface, the convex surface may be allowed to be restored to the former position, which means the attracting fixation cannot be effected in a stable manner. That is, if, to adjust one member to a predetermined angle with respect to the other member, the convex surface is caused to slide with respect to the concave member so that the central portion of the convex surface shown in FIG. 9 may move to a desired position, the convex surface returns to the former position, making it impossible to perform the mounting angle adjustment as desired.
Thus, in the universal joint constructed as described above, it is rather difficult to “freely” adjust the mounting angle of one member with respect to the other member.
It is accordingly an object of the present invention to improve the construction of the universal joint as described above and to provide a universal joint allowing the mounting angle of one member with respect to the other member to be freely adjusted and an image display unit to which such a universal joint is applied.